1. Field of the Invention
The present invention relates to a noise-free cascode circuit which is compatible with monolithic integrated circuit realization, and more particularly to a cascode circuit wherein the cascode element is a common-source junction field effect transistor which renders the circuit virtually noise-free and a current divider means for bypassing the majority of the current flowing in the input stage past the cascode JFET for significantly reducing the die area required for implementing the circuit in a monolithic integrated circuit.
2. Description of the Prior Art
The cascode circuit or sub-circuit employing two bipolar transistors has been in use for many years. The principal attributes of the cascode configuration are that the output resistance is very high and that no high frequency feedback occurs from the output back to the input through the parasitic capacitance or collector-base capacitance of a transistor such as occurs in the common-emitter configuration or the like. The high input impedance that is attainable particularly useful in obtaining power supply desensitization in bias reference supplies and in achieving large amounts of voltage gain in a single amplifying stage with an active transistor load.
Another significant problem with single state amplifier circuits and the like is that capacitance limits the speed at which voltages within a circuit can swing, owing to the finite driving impedance or current. When a capacitance is driven by a finite source resistance, RC exponential charging behavior is seen, whereas a capacitance driven by a current source leads to slew-rate-limited wave forms. As general guidance, reducing the source impedance and load capacitances and increasing the drive currents within a circuit will speed things up. However, there are some subtleties connected with feedback capacitance and input capacitance which deserve a look. The most serious of the problems is that of junction capacitance. The output capacitance C.sub.L forms a time constant with the output resistance R.sub.L to produce a rolloff starting at some frequency R=1/2.pi.R.sub.L C.sub.L. The same is true for the input capacitance in combination with the source impedance R.sub.s. The collector-base capacitance C.sub.cb is another matter. The amplifier has some overall voltage gain G.sub.v so even a small voltage wiggle at the input results in a greatly magnified wiggle and inverted output of the collector. This means that that signal source sees a current through C.sub.cb that is equal to an output voltage gain plus 1 (G.sub.v +1) times as large as if the collector-base capacitance C.sub.cb were connected from base to ground such that for the purpose of input rolloff frequency calculations, the feedback capacitance behaves like a capacitor of value C.sub.cb (G.sub.v +1) where G.sub.v is the overall voltage gain of the amplifier. This effective increase of C.sub.cb is known as the Miller effect. It often dominates the rolloff characteristic of amplifiers because a typical feedback capacitance will look like many times its actual value to ground. There are several methods available to minimize the effect of the Miller effect and one of the most effective methods found to date is to use a cascode element in the circuit. Still another problem in prior art circuits including those of a typical single stage amplifier include the problem that the addition of any device in the signal path between the input and output adds noise and this is true of the common cascode element as well. Therefore, even if a cascode element is added to reduce the Miller effect, a substantial increase in noise may occur to offset the desirability thereof.
However, even cascode circuits wherein a JFET transistor is used for both the signal amplifier stage and the cascode element, considerable problems exist particularly where it is desirable to be able to reduce the circuit combination or sub-circuit to a monolithic integrated circuit since the JFET generally requires a very large and relatively inefficient die area in order to carry the required input current thus rendering it virtually uneconomical to reduce the JFET cascode combination to a monolithic integrated realization.
These and the other problems of the prior art are solved in the circuits of the present invention which provide an extremely simple means for producing a noise free cascode circuit or subcircuit capable of being reliably and efficiently implemented as a monolithic integrated circuit.